In semiconductor manufacturing, technology and competition continue to contribute to shrinking die size and increased wafer size which result in a large number of die per wafer. In order to fabricate a high percent of reliable devices across the wafer, the uniformity of a film across the wafer deposited at any given process step becomes critical. The method used to form thin films must be economical with the resultant films exhibiting such characteristics as having a uniform thickness and good step coverage, high purity and density, good electrical properties, and excellent adhesion to name a few. Conventional approaches to deposit films has been by the use of one of the many chemical vapor deposition (CVD) processes.
A basic CVD process consists of the following sequence of steps: 1) a given composition and flow rate of reactant gases and diluent (or carrier) inert gases are released into a reaction chamber; 2) the gas species reactants are adsorbed on a silicon wafer substrate; 3) the loosely bonded atoms migrate across the substrate and cause filmforming chemical reactions, and 4) the gaseous by-products of the reaction are desorbed and removed from the reaction chamber. The chemical reactions that lead to formation of a solid material may be either heterogeneous (reaction occurs only on the wafer surface) or homogeneous (reaction occurs in the gas phase). Heterogeneous reactions are desirable as the reaction occurs on a heated surface and therefore can be controlled to produce good quality films.
Homogeneous reactions are less desirable as they form gas phase clusters of material (gas phase nucleation) which results in poor adhesion and low density films, as well as create particulates in the reaction chamber.
Two basic CVD reactor types used to deposit films are the hot wall reactor, using low pressure (typically 1 Torr or less) and high temperature (600.degree. C. or greater) and the cold wall reactor, using atmospheric pressure and a low temperature (&lt;600.degree. C.). Both reactor types have their advantages and disadvantages.
The main advantages of the cold wall reactor are its simple reactor construction, a fast deposition rate and a low deposition temperature. The main disadvantages of the cold wall reactor include poor step coverage and gas phase nucleation.
Additionally, the cold wall reactor may be of the plasma enhanced type wherein the chemical reaction is further promoted by heating the reactor with an rf generator, thereby creating free electrons (plasma) within the discharge region. The plasma enhanced CVD system produce films with desirable properties of good adhesion, good step cover and adequate electrical properties, to name a few.
A hot wall reactor provides deposited films with excellent purity and uniformity, while maintaining conformal step coverage. However, to produce this quality film, the hot wall reactor must use a high deposition temperature while the deposition rate is low. In industry, the advantages of the low pressure hot wall reactor out weigh its disadvantages, thus allowing the hot wall reactor to become the most widely used method for depositing films such as polysilicon and silicides.
A typical low pressure hot wall reactor is depicted in FIG. 1. Normal operation constitutes heating reactor chamber 1 and wafer boat 2 to a desired temperature by a heat source 3. The desired deposition pressure is set and controlled by pressure sensor 4, pressure switch 5 and pressure control valve 6. Reactor chamber is then flooded with a source gas and a small volume of a carrier gas is then input into the reactor chamber in order to set the deposition pressure. The source gas and carrier gas react to form a film on the heated wafer's substrate and the resultant gas is vented through exhaust 7 with the aid of blower 8.
The present invention may be used in the conventional CVD system mentioned above or it may be implemented in a CVD system as the one disclosed in pending application Ser. No. 704,533, also submitted by the same inventors of the present invention.
The best mode of the present invention uses the cold wall CVD system disclosed in pending application Ser. No. 704,533 and in addition, plasma is introduced into the system in order to enhance the deposition rate, lower the deposition temperature and to improve film quality. This plasma enhanced CVD system is used to deposit a titanium film, at a high deposition rate, that has excellent uniform density and step coverage while avoiding gas phase nucleation and coating of the reactor chamber walls.